Unit 8 Review Worksheet Solutions

Describe the structure of the Milky Way and the sun’s position in the galaxy.

Describe how the various regions of the Milky Way (disk, bulge, halo) differ from one another. (types of stars, motion of stars & amount of gas & dust)

The bulge and halo of the galaxy have very little gas and are full of old (Population II) stars. The gas and dust settled into the disk a long time ago. The stars orbit in random directions, much like comets in the Oort Cloud. The disk of the MW has lots of gas and dust. It contains both young and old Population I star. The stars in the disk orbit more like planets (all move around the center of the galaxy in roughly the same direction).

Describe where most star formation occurs in the Milky Way today.

The star formation occurs in the disk/spiral arms of the galaxy The star formation occurs in the disk/spiral arms of the galaxy. This is where most of the gas and dust can be found. Star formation is triggered by the spiral density waves.

4. Describe the evidence that supports the theory that there is a black hole at the center of our galaxy.

Astronomers have studied the motion of gas and stars at the center of the MW. They used Newtons version of Kepler’s 3rd law for stellar orbits to figure out that they orbit an unseen object with a mass between 3 – 4 million solar masses.

5. How do astronomers know there is more mass in the halo of the Milky Way than in the disk?

Astronomers know that there’s more mass in the halo because stars in the outer part of the galaxy are orbiting much faster than expected. They can tell by looking at the rotation curve, which is flat.

6. Describe the characteristics of spiral, elliptical and irregular galaxies.

Elliptical galaxies are featureless, round or oval in shape, contain only old (red) stars, and have no interstellar gas or dust. Spiral galaxies have a central bulge, flat disk, and spiral arms with active star formation and lots of gas and dust. Irregular galaxies have no structure, but very active star formation and lots of gas and dust.

7. What are Cepheid Variable stars and why are they important to astronomy?

Cepheid variables are pulsating stars Cepheid variables are pulsating stars. Their pulsation period is directly related to their true luminosity. The diagram below shows the graph of luminosity versus period for several types of variable stars. In general, the longer the period of the variable, the brighter the luminosity is. They are important because astronomers use them as “standard candles” (objects whose brightness is well determined) therefore they can be used for distance measurements.

8. How did Edwin Hubble measure the distance to the Andromeda galaxy?

Edwin Hubble observed Cepheid variables in Andromeda and used the period luminosity relationship to determine the distance.

9. State Hubble’s law and explain what it tells us about the motion of distant galaxies?

In equation form, Hubble’s Law is : V = Ho D In this equation, V is the velocity of a galaxy, H is Hubble’s constant, and D is the distance to the galaxy. A graph of Hubble’s law is linear, as shown. It tells us that distant galaxies are moving away from us at a greater speed.

10. Based on current estimates of the value of Hubble’s constant, how old is the universe?

The age of the universe is given by 1/Ho The age of the universe is given by 1/Ho. This estimate is that it’s between 12 and 16 billion years old. Most textbooks list the average age as 14 billion years.

11. How do observations of distant galaxies help us learn about galaxy evolution?

Observations of galaxies at different distances show us galaxies at different ages. The farther away a galaxy is the farther back in time we look. By comparing galaxies at different distances we get a better understanding of the stages of galaxy evolution.

12. What mechanism slowed the collapse of protogalactic clouds?

The collapse of protogalacitc clouds was slowed by the shock waves from the earliest supernovae.

13. What is a quasar?

A quasar (quasi-stellar object) is the extremely bright center of a distance galaxy that is probably powered by a supermassive black hole. Quasars are the most distant and most luminous objects we can see. Nasa image of Quasar 3C 273

14. Describe the common properties of active galaxies (AGs) and quasars. Explain how we think it gets its energy. Describe the structure of an AG and explain why orientation from our perspective is important

All AGNs have an extremely bright nucleus All AGNs have an extremely bright nucleus. They are small and powered by a supermassive black hole. The structure of an AGN includes the central black hole, which is surrounded by an accretion disk. The outer part of the accretion disk is surrounded by a dusty torus (which is shaped like a donut). Some AGNs also have jets of hot gas that shoot out from the center of the disk. The angle at which we see them (the orientation) determines which part of the structure we can see.

15. What does the universe look like on very large scales?

Galaxies clump together and are arranged in chains and sheets that surround huge empty regions called voids. For example, our galaxy is part of the local group, a small cluster of about 30 galaxies. The local group is part of the local supercluster.

16. What is the Big Bang Theory and what are the two key observational facts that led to the widespread acceptance of this model?

This theory says that the universe began about 13 This theory says that the universe began about 13.6 billion years ago from an infinitely dense, single point. The universe has been expanding and cooling ever since. We have detected the leftover radiation from the Big Bang (cosmic background radiation) The Big Bang theory correctly predicts the abundance of helium and other light elements (75 % H, 25 % He)

17. What is Olber’s Paradox and how does the Big Bang resolve Olber’s Paradox?

Olber’s Paradox states that in an infinite universe, filled with infinity of stars, we should see the sky bright as the Sun’s photosphere in all directions. However, night is dark because the universe is not infinitely old, therefore it can’t contain nearly enough stars to reach the brightness of the Paradox. In addition, the Big Bang weakens the radiation given off by distant objects (redshifted light has a lower energy) by the time we observe it.

18. Describe the candidates for dark matter.

Ordinary matter nicknamed Macho’s or massive compact halo objects Ordinary matter nicknamed Macho’s or massive compact halo objects. These would be things like failed stars (brown dwarfs) or black holes. Extraordinary matter nicknamed Wimps, or weakly interacting massive particles, much like massive neutrinos.

19. Explain what a standard candle is.

A standard candle is an object for which we know the true luminosity A standard candle is an object for which we know the true luminosity. Examples of standard candles include Cepheid variables and white dwarf supernovae. Because its true luminosity is known, a standard candle can be used to find the distance to the galaxy in which it’s found.

20. What is meant by the critical density of the universe?

The critical density is the precise density of matter that marks the dividing line between a universe that has enough mass to contract again and a universe that will continue to expand forever.

21. Summarize the steps of the distance ladder.

Describe the local group.

The local group is a small cluster of about 30 galaxies to which our galaxy belongs. The local group itself is part of a much larger group of galaxies called the local supercluster.

Explain why we think white-dwarf supernovae are useful for measuring cosmic distances.

These supernovae come from explosions of white dwarfs that have reached the mass limit of 1.4 solar masses. Since they reached the same mass limit, we expect them to have the same luminosity. This has been verified by observations of WD supernovae whose distances were known. The observations show that these supernovae have the same luminosity. Since we know the luminosity we can use their apparent brightness to find the distance to more distant supernovae using the luminosity – distance formula.

24. Describe our best current model for the Galactic Core, and how it was found. (new question)

In the constellation Sagittarius, there is a black hole of about 3 – 4 million solar masses, about 10 million kilometers across, almost 0.1 AU in size. It was originally discovered by the strong radio waves coming from its accretion disk. It’s called Sagittarius A, and it’s the strongest radio source in the sky. Since its discovery infrared observations of the rapid motions of very bright stars near it have allowed astronomers to estimate its mass.